What the animation shows
The animation and the step-by-step video above use your numbers. Here is the same walkthrough written out for the default example.
What you enter
- Length: 10 ft.
- Width: 10 ft.
- Thickness: 4 in. Patios and walks are commonly 4 in; driveways 4–6 in. Follow your plans or local code.
- Number of identical slabs: 1.
- Extra for waste and spillage: 10 %. Typical allowance is 5–10%. Use more for uneven subgrade.
- Bag size: 80 lb bag. Yields used: 40 lb ≈ 0.30 ft³, 50 lb ≈ 0.375 ft³, 60 lb ≈ 0.45 ft³, 80 lb ≈ 0.60 ft³.
What happens, step by step
- Concrete fills the forms 4 in deep on a gravel base.
- A screed board rides on the forms to level the top.
- The volume is length × width × thickness.
- 10% extra gives 1.36 cu yd, or 62 bags of 80 lb.
The answer: concrete to order (with waste) is 1.36 cu yd.
How it’s calculated
- Volume (ft³) = length (ft) × width (ft) × thickness (ft) × quantity
- Cubic yards = ft³ ÷ 27
- Order quantity = volume × (1 + waste %)
- Bags = order volume ÷ bag yield, rounded up
Worked example
A 10 ft × 10 ft patio, 4 in thick, with 10% waste and 80 lb bags.
Inputs
- Length
- 10 ft
- Width
- 10 ft
- Thickness
- 4 in
- Number of identical slabs
- 1
- Extra for waste and spillage
- 10 %
- Bag size
- 80 lb bag
- Ready-mix price per cubic yard
- 0 $/yd³
Results
- Concrete to order (with waste)
- 1.36 cu yd
- Net concrete volume
- 1.23 cu yd
- Or bags of pre-mixed concrete
- 62 bags
Assumptions and limits
- The slab is a rectangle with a uniform thickness.
- Subgrade is level; low spots increase the real volume.
- Bag counts use typical manufacturer yields for pre-mixed concrete. Check the yield printed on your bag.
- Rebar, mesh and forms are not deducted (their volume is negligible).
Verify the result against the applicable local code, adopted code edition, manufacturer instructions and project requirements. See our methodology.
Questions
How much concrete do I need for a 10×10 slab?
At 4 inches thick, a 10 × 10 ft slab needs 33.3 ft³, which is about 1.23 cubic yards before waste. With 10% extra, plan on about 1.36 yd³ or 62 bags of 80 lb mix.
How much extra concrete should I order?
Most contractors add 5–10%. Uneven ground, soft subgrade or long forms justify the higher end. Running short mid-pour is far more costly than a small surplus.
What is a cubic yard of concrete?
A cubic yard is 3 ft × 3 ft × 3 ft = 27 cubic feet. Ready-mix is sold by the cubic yard.